The present invention relates to a screw vacuum pump comprising two shafts and two rotors secured to the shafts, whereby each rotor has a central hollow chamber provided with devices for guiding a coolant flow.
Screw vacuum pumps of this kind are known from the German patent applications 197 45 616, 197 48 385 and 198 00 825. Since these are operated in a dry manner (without coolant or lubricant in the pump chamber), there exists the problem of dissipating the heat produced during operation caused chiefly by compressing the pumped gases.
In the instance of the screw vacuum pump known from DE-A-197 45 616 with a cantilevered rotor, both rotor and shaft are equipped with a pocket hole which is open towards the bearing side. A central coolant pipe affixed to the casing extends into the pocket hole, said coolant pipe being guided out of the bore on the side of the bearing and opening out on the rotor side just in front of the end of the pocket hole. With the aid of a coolant pump, the coolant is pumped through the central pipe into the bore. It flows back via the annular chamber between the stationary coolant pipe and the rotating inner wall of the pocket hole. The diameter of the bore in both the rotor and the shaft is relatively small so that the surfaces over which the coolant flows are also small. Moreover, shearing forces occur between the pipe fixed to the housing and the rotating wall of the hollow chamber, producing in the coolant unwanted friction and thus an increase in the temperature. The effectiveness of the desired cooling facility for the rotors is limited for these reasons.
In DE-A-197 48 385 two cooling methods are disclosed. In the instance of a first solution each of the rotors is equipped with a hollow chamber open on one side, into which the coolant is injected. Owing to the rotation, a film forms on the inside of the rotor said film flowing back to the opening of the hollow chamber. In the instance of a film cooling arrangement of this kind there exists the danger of the film breaking down so that the desired cooling effect is interrupted. In addition, it is proposed to equip the hollow chamber in the rotor with conical sections so as to be able to increase the dwell time of the coolant in the hollow chamber of the rotor and thus also influence the amount of heat dissipated. In the instance of such conical sections which support pumping of the flow, however, there all the more exists the danger of the coolant flow breaking down. Finally film cooling arrangements have the general disadvantage, that a stationary flow profile forms, in which that part of the film which is close to the wall flows much more slowly compared to the section of the film further away from the wall. The area close to the wall thus practically forms an isolating layer hampering heat dissipation. For the purpose of removing this disadvantage it has already been proposed to provide obstructions in the flow so that the stationary flow profile is interrupted by turbulence. Thus an exchange of heat can be attained between the film sections close to the wall and those further away from the wall. However, installing obstructions is involved and in all slows down the velocity of the coolant flow.
DE-A-198 20 523 discloses a cooling system similar to the one detailed above. Cooled oil is injected into a hollow section of the shaft. Due to centrifugal forces the oil is displaced outwards to the inner wall of the hollow chamber extending conically in the direction of the discharge side.
In a further embodiment disclosed in DE-A-197 48 385, the coolant flows through an annular slot between rotor shaft and a bearing base extending into the rotor""s hollow chamber. As to cooling of the rotor itself, a coolant flow of this kind has very little effect.
DE 198 00 825 discloses a screw pump with entirely hollow rotors. Coolant/lubricant is continuously supplied to, respectively discharged from the hollow chambers. As to the bearings which are arranged on the side where the coolant is discharged, a kinematic reversal has been implemented, i.e., they have a stationary inner ring and a rotating outer ring. The hollow chambers may be designed to be cone-shaped (widening in the direction of the flow) or may have an inner pumping thread. In the instance of such a screw pump there forms a coolant film which requires high rotational or circumferential velocities. The already mentioned danger of the coolant film breaking down exists. Monitoring facilities for an even spread of the coolant quantity to both rotors is, for this reason, absolutely recommended. Also, the necessity detailed of producing turbulence in the film exists, in order to attain an effective cooling effect. Finally equipping a rotor with a conical hollow chamber has several disadvantages: the conical hollow chamber is difficult to manufacture. In the instance of a cantilevered rotor on the delivery side and feeding in the coolant on the suction side of the rotor, the mass of the rotor in the area distant from the bearing is large. Correspondingly, when employing a cantilevered rotor on the delivery side the design needs to be involved. Finally, the fact that the coolant needs to be discharged relatively far out on the delivery side (at a great radial distance), limits the design options available.
DE-A-198 00 825 discloses a further embodiment, in which the rotors are each cantilevered on a shaft stub which extends into a hollow chamber in the rotor, said chamber being only open on the bearing side. The disadvantages detailed above also apply to this embodiment.
In the instance of all cooling systems detailed, there exists in addition the disadvantage that cooling is not performed in a counterflow. The coolant is in each instance supplied to the suction side of the rotor and not to the delivery side which is exposed to a significantly greater extent to the heat produced within the pump.
It is the task of the present invention to equip a pump of the aforementioned kind with an effective cooling arrangement permitting the pump to be manufactured in a simple, compact and cost-effective manner.
This task is solved through the characterising features of the patent claims.
It has been found that the cooling arrangement in accordance with the present invention in which the coolant flows through a relatively narrow, preferably cylindrical slot in the rotating system at sufficient velocity, has an unexpectedly good cooling effect, particularly since the annular slot can be arranged far to the outside, i.e. in the immediate vicinity of the root circle of the rotor""s profile. Since the coolant is not injected, any hollow cavities which might interrupt the cooling effect are not present. Finally there exists the advantage that the direction for the coolant flow may be selected freely so that there is no obstacle as to cooling by way of a counterflow. This results in an equalisation of the temperature spread so that on the delivery side and the suction side narrow rotor/casing slots can be maintained.
As to the selection for the thickness of the annular slot it is relatively narrow, 0.2 to 5 mm for example, preferably 0.5 to 2 mm, whereby the thickness of the slot also depends on the coolant employed, the oil commonly used in vacuum pumps, for example. Apparently it is important that the distance between the two boundary films close to the walls be relatively small so that they will mutually influence each other in a turbulent manner. A laminar flow not influenced by the boundary layers keeping these separated and impairing the transfer of heat is apparently not present or is of negligible thickness.
In order to effectively cool the rotors, the velocity of the coolant (again depending on the type of coolant employed) must be sufficiently high. Flow velocities in the order of 0.1 to 1 m/s, preferably 0.3 to 0.7 m/s have been found to be expedient in the instance of cooling oil. With the known oil supply pumps, be they centrifugal, gear or similar pumps, the required pressure differences can be generated.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.